The fabrication of semiconductor devices typically involves the formation of large numbers of integrated circuits or die on a silicon substrate or wafer. Fabrication of each die typically involves forming multiple layers over the semiconductor substrate and incorporating various impurities into the layers as well as the semiconductor substrate. The patterns used to form these layers and define regions in which impurities are incorporated are typically created using lithographic techniques.
The lithography process typically involves applying a photoresist layer (e.g., SiO.sub.2) over the surface of the semiconductor wafer and exposing the photoresist layer to a patterned light source. The light source is typically patterned using an exposure tool, such as a mask or reticle. The exposure tool typically contains clear and opaque features which generally define the pattern to be created in the photoresist layer. The exposed photoresist is then developed and regions of the photoresist are dissolved such that the pattern is transferred to the photoresist layer. The exposed regions of the underlying semiconductor wafer layer are then processed by, for example, etching the exposed wafer layer, depositing a material on the exposed wafer layer, or doping the exposed wafer layer. For a given die, the photolithography process may be used a number of times as layers are formed over layers to form the ultimate semiconductor device structure.
A number of different lithographic techniques may be used to project patterns onto a wafer. One common system for exposing the semiconductor wafer is a step-and-repeat system or stepper system. A stepper system generally includes an optical system having a light source and a reticle and a stepper for stepping or indexing the semiconductor wafer under the optical system. The reticle typically includes a pattern having one or more die patterns each of which are used to form a layer of a different die on the semiconductor wafer. In use, the semiconductor wafer is exposed by the optical system and the pattern on the reticle is transferred to the semiconductor wafer. The wafer is then stepped or indexed by the stepper to an adjacent field of the substrate and the adjacent field is exposed to the reticle pattern. Care is typically taken to prevent reticle flashes from overlapping one another. After the entire wafer is exposed, the photoresist is then developed to remove portions of the photoresist and prepare the wafer for subsequent processing, such as etching deposition or doping. Thereafter, the semiconductor may be returned to the stepper (or another stepper) for exposure of the semiconductor wafer to another reticle pattern.
Each of the die on a semiconductor wafer are typically separated by regions of the semiconductor wafer referred to as scribe lines. Scribe lines are typically about 100-200 microns wide and are used to separate adjacent die such that the individual die may be cut from the semiconductor wafer. Scribe lines are typically formed incorporating gaps between the die patterns of the reticle pattern used to form the die and by incorporating gaps between the die of adjacent fields. Test structures are typically formed within the scribe lines of a semiconductor wafer. A typical test structure is formed by providing a test structure pattern within a gap between the die patterns of a reticle pattern. When a field is exposed using such a reticle pattern, a pattern for a layer of the test structure is transferred to the wafer. As each test structure is associated with a reticle, when formed on the semiconductor wafer, these test structures are typically isolated from one another, i.e., they are isolated to their corresponding reticle field. When the wafer is cut to separate the individual die, the test structures are obliterated.